Embodiments of the invention relate generally to a switching system for On-Off switching of a current in a current path, and more particularly to micro-electromechanical system (MEMS) based switching devices.
Relays are electrically operated switches used to selectively control the flow of current between circuits so as to provide electrical isolation between a control circuit and one or more controlled circuits. Various types of relays are known and may be utilized based on the system and environment in which the relay is implemented, with electromechanical relays and solid-state relays being two common types of relays.
Electromechanical relays are switching devices typically used to control high power devices. Such relays generally comprise two primary components—a movable conductive cantilever beam and an electromagnetic coil. When activated, the electromagnetic coil exerts a magnetic force on the beam that causes the beam to be pulled toward the coil, down onto an electrical contact, closing the relay. In one type of structure, the beam itself acts as the second contact and a wire, passing current through the device. In a second type of structure, the beam spans two contacts, passing current only through a small portion of itself. Electromechanical relays beneficially provide the ability to withstand momentary overload and have a low “on” state resistance. However, conventional electromechanical relays may be large in size may and thus necessitate use of a large force to activate the switching mechanism. Additionally, electromechanical relays generally operate at relatively slow speeds and, when the beam and contacts of the relay are physically separated, an arc can sometimes form therebetween, which arc allows current to continue to flow through the relay until the current in the circuit ceases, while damaging the contacts.
Solid-state relays (SSR) are an electronic switching device that switches on or off when a small external voltage is applied across its control terminals. SSRs include a sensor which responds to an appropriate input (control signal), a solid-state electronic switching device (e.g., thyristor, transistor, etc.) which switches power to the load circuitry, and a coupling mechanism to enable the control signal to activate the switch without mechanical parts. SSRs beneficially provide fast switching speeds compared with electromechanical relays and have no physical contacts to wear out (i.e., no moving parts), although it is recognized that SSRs have a lower ability to withstand momentary overload, compared with electromechanical contacts, and have a higher “on” state resistance. Additionally, since solid-state switches do not create a physical gap between contacts when they are switched into a non-conducting state, they experience leakage current when nominally non-conducting. Furthermore, solid-state switches operating in a conducting state experience a voltage drop due to internal resistances. Both the voltage drop and leakage current contribute to power dissipation and the generation of excess heat under normal operating circumstances, which may be detrimental to switch performance and life and/or necessitate the use of large, expensive heat sinks when passing high current loads.
Micro-electromechanical systems relays (MEMS relays) have been proposed as an alternative to SSRs with most of the benefits of conventional electromechanical relays but sized to fit the needs of modern electronic systems. In such MEMS relays, isolation is needed between the control terminals and power terminals of the MEMS relay—i.e., between a control side of the relay and a power side of the relay that includes a MEMS switch and an auxiliary circuit. In addition, at the power side, electronic circuits are needed to drive the MEMS relay (e.g., MEMS driver), which requires high gate voltage, and a logic circuit is needed to control the switching signals for the MEMS switch and auxiliary circuit. Therefore, it is required to transfer control signals (On-Off signals) and power across an isolation barrier. Traditionally, this isolation is via use of an isolated power supply that powers the high side circuit and an optocoupler that transfers the On-Off control signal; however, the use of such components to provide isolation and the transfer of control signals increases the cost of the MEMS relay and the printed circuit board area taken up thereby.
Therefore, it is desirable to provide a MEMS relay circuit that provides isolation between a low voltage control side and high voltage power side, provides for transfer of power from the low voltage control side to the high voltage power side, and provides for the transmission of control signals from the low voltage control side to the high voltage power side. It is further desirable that such a MEMS relay circuit provides such functionality using low cost electronic circuits that reduce the cost and size of the MEMS relay circuit.